Femtocell base stations in a Long Term Evolution (LTE) communication network (otherwise known as Home evolved Node Bs—HeNBs—or Enterprise evolved Node Bs—EeNBs) are small, low-power, indoor cellular base stations for residential or business use. They provide better network coverage and capacity than that available in such environments from the overlying macrocellular LTE network. In addition, femtocell base stations use a broadband connection to receive data from and send data back to the operator's network (known as “backhaul”).
As femtocell base stations make use of the same air interface as macrocell base stations in the macrocellular network, and as they are located within the coverage area of one or more macrocell base stations in the macrocellular network, it is necessary to ensure that uplink transmissions from mobile devices (otherwise known as User Equipments—UEs) to the femtocell base station do not interfere substantially with uplink transmissions from other mobile devices to the macrocell base stations.
It has been suggested that this interference can be mitigated by placing a cap on the power that a mobile device can use to transmit to the femtocell base station. This cap is determined from an estimated path loss to the neighbouring macrocell base station with the strongest signal, such that, when the mobile device is transmitting to the femtocell base station at the capped power level, the increase in noise experienced at the macrocell base station is limited to a specified amount (measured in decibels, dB).
There is a problem with this approach in that the maximum permitted transmission power level only considers the noise increase caused by an individual mobile device. In order to maintain the noise rise at the macrocell base station to the specified fixed amount, the cap needs to be a function of the number of mobile devices that are transmitting to femtocell base stations (i.e. an increasing number of mobile devices transmitting to femtocell base stations should lead to a lower maximum permitted transmission power).
Hence, there is a need for an alternative approach to reducing the interference caused by mobile devices transmitting to femtocell base stations.